The principle governing the operation of the read sensor in a magnetic disk storage device is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance). Magneto-resistance can be significantly increased by means of a structure known as a spin valve. The resulting increase (known as Giant magneto-resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of the solid as a whole.
The key elements of a spin valve structure are shown in FIG. 1. Seen there, on substrate 10, is exchange pinning layer 7, made of PtMn or NiMn and having a thickness of about 100–300 Å. Layer 6 is a pinned layer of CoFe or CoFe/NiFe laminate, having a thickness of 20–200 Å. (for a synthetic spin valve the layer would be CoFe/Ru/CoFe). Layer 5 is a spacer layer of Cu, Au or Ag with a thickness of 5–25 Å while layer 4 is the free layer, of CoFe or CoFe/NiFe laminate (thickness 20–200 Å).
Although layers 4–7 are all that is needed to produce the GMR effect, additional problems remain. In particular, there are certain noise effects associated with such a structure. As first shown by Barkhausen in 1919, magnetization in iron can be irregular because of reversible breaking of magnetic domain walls, leading to the phenomenon of Barkhausen noise. The solution to this problem is to provide operating conditions conducive to single-domain films for MR sensor and to ensure that the domain configuration remains unperturbed after processing and fabrication steps. This is most commonly accomplished by giving the structure a permanent longitudinal bias provided, in this instance, by layer 3 which is a hard bias material such as Cr/CoPt or Cr/CoCrPt (where Cr is 0–200 Å), CoPt or CoCrPt (100–500 Å). Layer 2 is a protection layer of Ta or Ru with a thickness of 1–30 Å.
Of particular interest for the present invention is layer 1 from which the input and output leads to the device are fabricated. An example of a lead material is Ta/Au/Ta, where Ta is 20–100 Å and Au is 100–500 Å. One of the major problem in Lead Overlay (LOL) design is that the magnetic read track width is wider than physical read track width. This is due to high interfacial resistance between the lead and the GMR layer if integration is done with conventional metallurgy. This is symbolized in FIG. 1 by current flow along path B instead of along the ideal path C.
In FIG. 2A we illustrate the first of two prior art methods that have been used to form the leads. With layers 3 through 7 in place, a lift-off mask is formed. This comprises two layers 21 and 22. Both layers are light sensitive and therefore patternable in the usual way. Top layer 22 is conventional photoresist but lower layer 21 is a material that is readily etched away. Consequently, when a layer is laid down over this structure, as shown in FIG. 2B, part of this layer (23A) deposits onto the spin valve top surface and part of it (23B) deposits onto upper pattern 22. Then when a solvent to remove part 21 is applied, the latter soon dissolves and part 22, including layer 23B, lifts off and is removed, leaving behind two leads separated by the original width of 22, as shown in FIG. 2C.
The problem with this approach is that there is a degradation of electrical conductance at the tip of the lead arising from the resist shadowing leading to poor track width definition for extremely narrow track widths. Area “A” marked in FIG. 2C indicates the area of poor track width definition and lowered electrical conductance area.
An improved fabrication process has been reported by Tanaka et al. and is illustrated in FIG. 3A. A dry etch is used to define the separation between the leads (track width 36). However, this approach also results in wider magnetic read width than physical read width because a relatively thick etch stop layer 32 is required in this approach to properly define leads 33 without etching into the GMR sensor. This etch stop layer generally consists of slow dry etch materials such as Ta, Cr, W, Ti, or their alloys. These materials are often high in electrical resistivity and the resulting high interface resistance prevents the electrical current from flowing into the very edge of the lead—the current flow path that is obtained is the one marked as B in FIG. 3C. The ideal current flow path is marked by G in the FIG. 3C.
The present invention discloses a process to manufacture a structure that allows current flow through path C, This results in much smaller magnetic/physical read track width difference.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,188,495, Wiitala describes a lead process for a SV MR. In U.S. Pat. No. 6,118,621, Ohsawa also show a lead process. (Shouji et al. discloses MR heads with different shaped leads in U.S. Pat. No. 5,907,459 while in U.S. Pat. No. 5,761,013, Lee et al. discuss leads and routing.